1. Field of the invention
The invention lies in the field of protection systems for networks, such as passive optical networks. More in particular, it relates to a passive optical network having a protection configuration for applying a protection principle based on multiple accessibility, such as the principle of dual homing.
2. Prior art
Developments are increasingly going in a direction of applying passive optical networks (PONs) to the access network, with optical-fibre connections being pulled through to near the subscriber (FTTC fibre to the curb! or FTTH fibre to the home!). Configurations for such an access network based on a PON for providing narrow- and wide-band communication between a main station and a large number of linked-up subscribers are known per se, such as, e.g., from reference 1! (see below under C. for more details with respect to the references). Said known network comprises a tree-shaped branching of optical connections, hereinafter called access network, which is provided with a large number (up to approx. 2000) of branchings (or splittings-up) at the subscriber connection side, and a trunk-shaped open feeder, hereinafter called feed network, to an access node of the optical network, where a main station is located. In the access network, the splitting up is effected in two stages, i.e., a first stage (1:16) directly at the connection to the feed network and a second stage (1:128) nearer the subscriber connections. The lengths of the optical connections in the access network are relatively short (&lt;10 km). The length of the feed network may rather vary (0-100 km), depending on the position of the main station. To be capable, at the subscriber connection side, of detecting signals of sufficient strength, signal amplification is required in view of the high degree of splitting-up in the access network, and depending on the length of, in particular, the feed network. Therefore, in the known configuration there are included, at two locations in the optical connections, optical signal amplifiers, namely, a feeder repeater halfway through the feed network and a splitter repeater in the coupling of the feed network to the access network. Such an optical network is vulnerable, however, when optical connections fail, namely, all the more vulnerable in the event of a larger number of connections and a longer feed network. Particularly a fibre or cable breakage in the feed network will have serious consequences. To minimize the consequences of failure of connections in the feed network as a result of cable breakage or equipment failure, the entire network might be duplicated. This is very costly, however, and not strictly necessary. In fact, as a cable breakage or equipment failure in the network occurs at a location closer to the subscriber side, where the access network is further split up, the size of the consequences will decrease. The known technique makes use thereof by applying a protection configuration in which the feed network and the first stage of the access network are duplicated, and the accepted principle of dual homing (see, e.g., reference 2!) is applied, with the redundant feed network leading to a second access node, which offers a second main station access to the network or also offers the same main station a second access to the network. An optical network having such a protection configuration, however, proportionally is still costly, especially in geographical situations in which the probability of failure occurring in the duplicated part of the network, and the redundant part of the network being actually used, is slight.
In reference 3! there are disclosed self-healing architectures for "fiber-in-the-loop" (FITL) networks having a relatively small number of connections. In a first version, every connection in a distribution area is also accessible from a main station by way of a protection fibre connection which runs through an adjacent distribution area. In a second version, an operational fibre connection in a distribution area is also used as a protection fibre connection for an adjacent distribution area, with a WDM wavelength division multiplex! technique having a separate wavelength, being applied to the protection signal transport. Said known self-healing architectures are not or hardly suitable for tree-shapedly branched optical networks having large numbers of network connections.
Reference 4! describes a protection switching system which comprises pairs of (electrical) telecommunication modules for processing or switching telecommunication signals. Each pair comprises an operational module and a standby module. The system further comprises monitoring and switching means for switching over to the standby module upon failure of the corresponding operational module with, as long as the standby module is standby, the latter monitoring the operation of the operational module. The pairs of telecommunication modules may be connected to one another by means of optical-fibre connections. If, according to such a protection technique, an optical network, such as the PON described above, is provided with a protection configuration with, e.g., the feed network and the first stage of the access network forming the duplicated modules, for said optical network there also apply the drawbacks already referred to.